Definition of arch-enemy in English:

noun

The designer who created the game, Yuji Naka - president and CEO of Sonic Team - is busy these days doing what would have been anathema a year ago: making a Sonic variant for a former arch-enemy, Nintendo.

Everyone knows that a hero is only as interesting as his arch-enemy and the bad guy in this film, while amusing, isn't particularly interesting.

One would think that a leader who has brought about a historic detente with an arch-enemy would be cheered and feted by his people.